(Meth)acrylic acid/itaconic acid copolymers, their preparation and use as antiscalants

ABSTRACT

Substantially homogeneous (meth)acrylic acid/itaconic acid copolymers of number average molecular weight of 500 to 7000 are prepared by copolymerizing in aqueous solution 5 to 90 mole percent acrylic or methacrylic acid monomer with 95 to 10 mole percent itaconic acid monomer at 80° to 120° C. in the presence of a polymerization initiator, the acrylic or methacrylic acid monomer and at least half of the initiator being added separately and continuously to the itaconic acid monomer throughout the polymerization period. The copolymers are employed at a level of from about 0.1 to 100 ppm for prevention of alkaline calcium and magnesium scale formation, such as during seawater evaporative desalination.

This is a continuation of application Ser. No. 06/401,055, filed on July22, 1982 U.S. Pat. No. 4,485,223 which is a continuation-in-part ofapplication Ser. No. 06/316,365, filed on Oct. 29, 1981, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to novel copolymers of itaconic acid with acrylicor methacrylic acid, a process for their preparation and their use asantiscalants.

Aqueous systems containing dissolved mineral salts are commonly employedin operations such as heating, cooling and evaporative distillationwhich involve heat transfer. The salts often become insoluble during theoperation and are deposited as scale on the heat transfer surfaces,resulting in reduced heat transfer and eventual failure of theequipment. In general, the mineral scale salts are derived from alkalineearth and other metal cations such as calcium, magnesium, iron and lead,and from anions such as bicarbonate, carbonate, hydroxide, sulfate andphosphate. Many factors influence scale formation and include, forexample, the pH of the water, the nature of the dissolved salts and thetemperature of the operation.

Numerous additives have been proposed as inhibitors of mineral scaling.Of special importance are those additives known as threshold scalecontrol agents which exhibit scale inhibition at extremely low additiveconcentrations of even one part per million or less. Included among thethreshold scale control agents are acrylic and methacrylic acidhomopolymers and copolymers with such as maleic anhydride and fumaricacid. While these additives have proven to be effective at thresholdconcentrations, their preparation generally involves use of nonaqueoussolvent systems which are costly and which introduce the problem ofsolvent contamination of the polymer product. The need therefore stillexists for effective threshold scale control agents which can be simplyand inexpensively prepared in such as aqueous solvent systems.

(Meth)acrylic acid/itaconic acid copolymers have been prepared in thepast in aqueous systems, but the use of such copolymers as scaleinhibitors has not been disclosed. Thus, the (meth)acrylic acid/itaconiccopolymer of U.S. Pat. No. 3,308,067 is used as a detergent builder,while that of U.S. Pat. No. 3,366,509 is intended as a sizing agent andthe copolymer of U.S. Pat. No. 3,507,647 is employed in lithographicprinting. Published United Kingdom Application GB2054548A discloses theuse of a (meth)acrylic acid/itaconic acid copolymer in removing boilerscale. Both Marvel et al, J. Org. Chem., 24, 599 (1959) and Nagai et al,Kobunshi Kagaku, 17, 748 (1960) disclose methods of preparing(meth)acrylic acid/itaconic acid polymers but fail to mention a use forthe resulting copolymer. All of these disclosed preparations involve aconventional copolymerization employing an aqueous medium and standardinitiator system which results in copolymers of relatively highmolecular weight.

It is therefore a primary objective of the present invention to providea novel process for the preparation of substantially homogeneous(meth)acrylic acid/itaconic acid copolymers of controlled low molecularweight which are particularly effective as threshold scale inhibitors.

SUMMARY OF THE INVENTION

It has now been found that substantially homogeneous, low molecularweight copolymers of acrylic or methacrylic acid, or mixtures thereof,with itaconic acid may be readily prepared by the separate andcontrolled addition of the (meth)acrylic acid monomer and thepolymerization initiator to an aqueous solution of the itaconic acidmonomer.

Accordingly, the present invention entails a process for preparing asubstantially homogeneous (meth)acrylic acid/itaconic acid copolymer ofnumber average molecular weight of from about 500 to 7000 whichcomprises contacting in aqueous medium from about 95 to 10 mole percentitaconic acid monomer with from about 5 to 90 mole percent acrylic ormethacrylic acid monomer over a period of from about 2 to 8 hours at atemperature of from about 80° to 120° C. in the presence of from about 5to 20 mole percent polymerization initiator, based on the total amountof the monomers, the acrylic or methacrylic acid monomer and at leasthalf of the initiator being added separately and essentiallycontinuously throughout the period to the itaconic acid monomer insolution in the medium.

In preferred embodiments of the process, from about 95 to 60 molepercent itaconic acid monomer is copolymerized with from about 5 to 40mole percent methacrylic acid monomer and the initiator is a redoxsystem. Preferably, the itaconic acid monomer and from about 2 to 25percent of the initiator are initially dissolved in the aqueous mediumand the remainder of the initiator is introduced over the polymerizationperiod, and the resulting copolymer solution is maintained at thepolymerization temperature for from about 10 to 120 minutes followingthe addition.

In an especially preferred embodiment of the process, about 5 to 40 molepercent methacrylic acid monomer, from about 1 to 2 mole percent sodiumpersulfate and from about 6 to 12 mole percent sodium bisulfite areadded separately over a period of from about 3 to 5 hours to an aqueoussolution of from about 95 to 60 mole percent itaconic acid monomer andfrom about 0.02 to 0.04 mole percent ferrous sulfate at atmosphericreflux temperature, and the solution is held at that temperature forfrom about 45 to 75 minutes following the addition.

The present invention also entails a substantially homogeneous copolymerconsisting essentially of from about 5 to 90 mole percent acrylic ormethacrylic acid units and from about 95 to 10 mole percent itaconicacid units and having a number average molecular weight of from about500 to 7000. The copolymer has preferably from about 5 to 40 molepercent methacrylic acid units and from about 95 to 60 mole percentitaconic acid units; may be in the form of the alkali metal, ammonium oramine salt thereof; and is normally as an aqueous solution containingfrom about 1 to 60 weight percent of the copolymer.

The present invention further entails a method for the prevention ofscale formation in water containing scale-forming impurities, such as inseawater for desalination, which comprises mixing with the water aneffective amount of the copolymer, preferably at a concentration of fromabout 0.5 to 10 ppm. The method may also comprise addition to theseawater of an amount of such as sulfuric acid to neutralize preferablyfrom about 55 to 85 percent of the bicarbonate alkalinity of theseawater.

DETAILED DESCRIPTION OF THE INVENTION

The novel polymerization process of the present invention, as describedherein, provides for the first time essentially homogeneous copolymersof controlled low molecular weight from the copolymerization of majoramounts of acrylic or methacrylic acid with itaconic acid. In addition,these novel copolymers exhibit improved antiscalant activity over the(meth)acrylic acid/itaconic acid copolymers of the prior art, therebyresulting in their more efficient utilization during such as the flashevaporation of seawater in preparing potable water.

Homogeneous copolymers are defined by the art as compositions of narrowmolecular weight distribution. The distribution is readily determined bysuch analytical techniques as high pressure liquid chromatography (HPLC)and is essentially mononodal for homogeneous copolymers. Nonhomogeneouscopolymers, on the other hand, characteristically have binodaldistributions, particularly when the reactivities of the two monomersare very different as is the case with (meth)acrylic acid and itaconicacid. When determined by HPLC, the molecular weight distribution of thesubstantially homogeneous copolymers of the present invention isessentially mononodal with the ratio of the weight average molecularweight (MW_(W)) to the number average molecular weight (MW_(N)) beingless than 3.

In this process, from about 95 to 10 mole percent of itaconic acid iscopolymerized in aqueous medium with from about 5 to 90 mole percent ofeither acrylic acid or methacrylic acid in the presence of a specifiedamount of polymerization initiator and at specified temperatures andpolymerization times, the acrylic acid or methacrylic acid monomer andthe initiator being added separately and simultaneously to the itaconicacid monomer. Acrylic acid and methacrylic acid copolymerize inessentially the same manner with itaconic acid, and may therefore beinterchanged or mixed in the process to give products with essentiallythe same molecular weight and antiscalant characteristics for acopolymer of given (meth)acrylic acid/itaconic acid mole ratio.

Since the present process is conducted in aqueous medium, a considerablesavings in solvent and processing costs is realized, the problem ofsolvent contamination of the product does not exist, and the necessityof isolating the product from the final reaction solution is eliminated.The amount of water present during the polymerization is critical onlyto the extent that the reaction mixture remains a solution throughoutthe polymerization and is concentrated enough to fulfill the time andtemperature process parameters. Thus, the polymerization will normallybe conducted at a monomer concentration slightly below the aqueoussolubility of both itaconic acid and the resulting copolymer, generallyin the range of from about 0.75 to 100 grams water per gram totalmonomer. While the polymerization is preferably conducted in wateralone, the possibility of the presence of minor amounts of water-solublesolvent, if such is desired, is not excluded.

Any water-soluble, free-radical initiator may be used as thepolymerization initiator of this process. Suitable initiators includepersulfates such as sodium and potassium persulfate as well as redoxsystems. The redox system, such as ferrous sulfate/sodiumpersulfate/sodium bisulfite, is preferred. The polymerization initiatorshould be present in the amount of from about 5 to 20 mole percent basedon the total amount of the monomers. All or at least half of theinitiator is added, separately from the (meth)acrylic acid monomer,essentially continuously throughout the polymerization period.Preferably, from about 2 to 25 percent of the initiator is dissolvedalong with the itaconic acid in the aqueous medium and the remainder ofthe initiator is then introduced, as an aqueous solution, over thepolymerization period. The concentration of the initiator in the aqueousaddition solution is normally from about 5 to 25 weight percent.

The temperature and duration of the polymerization are influential indetermining the nature of the resulting copolymer. The polymerization istherefore limited to temperatures of from about 80° to 120° C. formonomer addition periods of from about 2 to 8 hours. Polymerization atatmospheric reflux is preferred. Since the presence of a minimum amountof monomer in the final product is desired, the final polymerizationsolution is generally maintained at the polymerization temperature for aperiod, preferably from about 10 to 120 minutes, following thecompletion of the (meth)acrylic acid monomer and initiator additionperiod.

By the selection of the above reaction parameters within the specifiedranges, essentially homogeneous (meth)acrylic acid/itaconic acidcopolymers with number average molecular weights of from about 500 to7000, preferably from about 1000 to 4000, are readily prepared. Thus,the lower molecular weights are obtained by employing more dilutesolutions at the higher temperatures with larger amounts of initiatorand shorter addition times. Conversely, the higher molecular weights arerealized in more concentrated solutions at the lower temperatures withsmaller amounts of initiator and longer addition periods.

Preferred embodiments of the instant process include those in which fromabout 5 to 40 mole percent methacrylic acid is copolymerized with fromabout 95 to 60 mole percent itaconic acid. In an especially preferredprocess, about 5 to 40 mole percent methacrylic acid, 1 to 2 molepercent sodium persulfate and 6 to 12 mole percent sodium bisulfite areadded separately over a period of about 3 to 5 hours to an aqueoussolution of about 60 to 95 mole percent itaconic acid and 0.02 to 0.04mole percent ferrous sulfate at atmospheric reflux temperature, and thepolymerization solution is held at the reflux temperature for anadditional 45 to 75 minutes following the addition.

The essentially homogeneous copolymer consisting essentially of fromabout 5 to 90 mole percent acrylic or methacrylic acid units and fromabout 95 to 10 mole percent itaconic acid units and having a numberaverage molecular weight of from about 500 to 7000 may be isolated insolid form from the final polymerization solution by standardtechniques, such as evaporation followed by drying. However, since thecopolymer will normally be added to aqueous systems, the finalpolymerization solution, as such, diluted or concentrated as desired,will generally be used without isolation of the copolymer product. Thissolution may also be neutralized to form an aqueous solution of thealkali metal, ammonium or amine salt of the copolymer. Alkali metalbases suitable for the neutralization include sodium hydroxide,potassium hydroxide and lithium hydroxide, while suitable ammonium andamine bases include ammonia, ammonium hydroxide, mono-, di-and trialkylamines having 1 to 5 carbon atoms in each alkyl group, pyridine,morpholine and lutidine.

The essentially homogeneous (meth)acrylic acid/itaconic acid copolymersof the present invention, while having general thresholdscale-inhibiting properties, are especially useful in the prevention ofalkaline calcium and magnesium scale formation in such processes as theevaporative desalination of seawater. In such use, the copolymer isadded to the water being treated in an effective amount, normally at alevel from about 0.1 to 100 ppm and preferably from about 0.5 to 10 ppm,to inhibit scale formation. In such addition, the aqueous copolymersolution at about 1 to 60 weight percent copolymer or salt equivalentthereof may be metered directly into the stream being treated, butpreferably is diluted with water to a concentration of about 0.1 to 1weight percent before being added. Evaluation of these essentiallyhomogeneous, controlled low molecular weight copolymers has shown themto be superior to the (meth)acrylic acid/itaconic acid copolymers of theprior art.

Other additives commonly used in combination with threshold scaleinhibitors are also effective with the novel inhibitors of the presentinvention. Especially useful are mineral acids, especially sulfuricacid, whereby part of the bicarbonate alkalinity present in such asseawater to be desalinated is neutralized; the extent of the bicarbonateneutralization is normally from about 30 to 85 percent, especially fromabout 55 to 85 percent. Foam control agents, corrosion inhibitors andoxygen scavengers, for example, may also be utilized with these novelscale inhibitors.

The following examples are merely illustrative and are not to beconstrued as limiting the invention, the scope of which is defined bythe appended claims.

EXAMPLE 1

91.1 g (0.70 mole) itaconic acid and 5.95 g (0.025 mole) sodiumpersulfate were dissolved in 100 ml distilled water contained in a 300ml round bottom flask equipped with stirrer, reflux condenser andnitrogen inlet. The system was purged with nitrogen and the solutionthen heated to reflux under the nitrogen atmosphere. Separate streams of25.8 g (0.30 mole) methacrylic acid and an aqueous solution of 17.9 g(0.075 mole) sodium persulfate in 60 ml water were added continuously bymeans of syringe pumps to the refluxing solution over a 5-hour period.Once the addition was completed, the solution was refluxed (103° C.) foran additional 60 minutes. The resulting solution was then distilledatmospherically to remove 30 ml water and produce a concentrate of 239.7g of an orange solution containing 58.4 weight percent solids. Thisrepresents a 99.5 weight percent product yield based on the combinedweight of the charged monomers and the sodium persulfate initiator.

The molecular weight distribution and chemical homogeneity of thepolymer product was determined by high pressure liquid chromatography(HPLC) on a series of three 25 cm 60 Å u-Porasil gel permeation columnsusing an acetate-phosphate buffer eluant adjusted to pH 7.4 with 1Naqueous sodium hydroxide, the Porasil columns being calibrated with1,2,3,4-butanetetracarboxylic acid and Goodrite K 732 and K 752polyacrylates of known molecular weight. This analysis indicated theproduct to be a homogeneous copolymer with a number average molecularweight of 1600 and a weight average molecular weight of 2940.

EXAMPLE 2

The preparation of Example 1 was repeated using various acrylicacid/itaconic acid and methacrylic acid/itaconic acid monomer ratios andvarious mole percents of sodium persulfate initiator to total monomer asindicated in Table I. Each preparation resulted in an essentiallyhomogeneous copolymer product having the indicated number and weightaverage molecular weights.

EXAMPLE 3

To the stirred reactor of Example 1 was charged 68 ml water, 0.08 g (0.3millimole) ferrous sulfate heptahydrate and 39.0 g (0.30 mole) itaconicacid. The reactor was purged with nitrogen and the resulting solutionwas heated to reflux (100° C.) while maintaining a positive nitrogenatmosphere. Separate streams of 3.69 g (15.0 millimole) sodiumpersulfate in 38 ml water, 9.92 g (95.0 millimole) sodium bisulfite in30 ml water and 50.4 g (0.70 mole) glacial acrylic acid were addedcontinuously over 3.5 hours to the refluxing solution. Once thesimultaneous additions were completed, the solution was maintained atreflux for an additional 60 minutes. The resulting yellow solutionweighed 205.6 g with a solids content of 52.8 weight percent,representing a 105.2 weight percent yield based on the chargedreactants. HPLC analysis of the product solution indicated the productto be a substantially homogeneous (meth)acrylic acid/itaconic acidcopolymer of number average molecular weight 2210 and weight averagemolecular weight of 4460.

The preparation was repeated using an itaconic acid/acrylic acid moleratio of 70/30 and a redox initiator of 1.5 mole percent sodiumpersulfate, 9.5 mole percent sodium bisulfite and 0.03 mole percentferrous sulfate, resulting in a 68.2 weight percent product yield of asubstantially homogeneous (meth)acrylic acid/itaconic acid copolymer ofnumber average molecular weight 900 and weight average molecular weightof 1910.

EXAMPLE 4

The preparation of Example 1 is repeated with the exception that 123.5 g(0.95 mole) of itaconic acid is copolymerized at 80° C. with 4.3 g (0.05mole) methacrylic acid in the presence of 47.6 g (20 mole percent)sodium persulfate. The glacial methacrylic acid and all of the sodiumpersulfate (as a 10 weight percent aqueous solution) are addedcontinuously as separate streams over an 8-hour period. Heating iscontinued for an additional 10 minutes once the additions are complete.A substantially homogeneous low molecular weight (meth)acrylicacid/itaconic acid copolymer similar to that of Example 1 is obtained.

COMPARATIVE EXAMPLE 1

A (meth)acrylic acid/itaconic acid copolymer (65/35 acrylicacid/itaconic acid) was prepared in water following the procedure of C.S. Marvel and T. H. Shepherd, Journal Organic Chemistry, Volume 24, page599 (1959). A copolymer of number average molecular weight greater than8000 as determined by HPLC was obtained in high yield.

COMPARATIVE EXAMPLE 2

(Meth)acrylic acid/itaconic acid copolymers (50/50 acrylic acid/itaconicacid and methacrylic acid/itaconic acid) were prepared following theprocedure described in Example 1 of U.S. Pat. No. 3,366,509. Themethacrylic acid/itaconic acid copolymer formed a rubbery gel in waterwhich hindered isolation and prevented scale control testing. Theacrylic acid/itaconic acid copolymer was obtained in essentiallyquantitative yield and possessed a number average molecular weight ofgreater than 10,000 as determined by HPLC.

EXAMPLE 5

Methacrylic acid/itaconic acid and acrylic acid/itaconic acid copolymersof the preceding examples were tested in a laboratory single-stage flashevaporator using a synthetic seawater to determine their scale controlperformances. The operation of the laboratory evaporator and the methodof testing was as described by Auerbach and Carruthers in Desalination,31, 279 (1979). The synthetic seawater was the "Standard SyntheticSeawater Composition" of the Office of Saline Water containing anaugmented bicarbonate content of 0.25 g/kg to increase the scalingpotential. The copolymer concentration in the testing was 3 parts permillion on an active solids basis.

Results of the testing are summarized in Table II. The percent depositedscale is calculated from the equation: ##EQU1## Copolymers which providelower "% deposited scale" at a given dosage afford superior performance.

                  TABLE I                                                         ______________________________________                                        Copolymerization of Acrylic Acid or Methacrylic Acid                          with Itaconic Acid Using Sodium Persulfate Initiator                                                Init.,.sup.(2)                                                                      Product                                               Co-       Mole    Mole  Yield,                                            Ex  polymer.sup.(1)                                                                         Ratio   %     Wgt %.sup.(3)                                                                        MW.sub.N .sup.(4)                                                                    MW.sub.W .sup.(5)                   ______________________________________                                        2A  IA/MAA    90/10   15    96.9   1310   2230                                2B  IA/AA     90/10   15    96.9   1550   2780                                2C  IA/MAA    70/30   10    99.5   1600   2940                                2D  IA/MAA    70/30    5    98.4   2640   4460                                2E  IA/MAA    50/50   15    96.2   1300   2410                                2F  IA/MAA    10/90   15    98.2   1330   2650                                ______________________________________                                         .sup.(1) IA = itaconic acid; MAA = methacrylic acid; AA = acrylic acid        .sup.(2) sodium persulfate initiator, mole % total monomer                    .sup.(3) solids as weight % of monomers plus initiator                        .sup.(4) number average molecular weight                                      .sup.(5) weight average molecular weight                                 

                  TABLE II                                                        ______________________________________                                        Antiscalant Scale Inhibition                                                  Copolymer       Example  % Deposited Scale                                    ______________________________________                                        IA/MAA, 90/10   2A       19.4                                                 IA/MAA, 50/50   2E       18.5                                                 IA/MAA, 10/90   2F       21.0                                                 IA/AA, 70/30    3        19.6                                                 IA/AA, 35/65    Comp 1   24.3                                                 IA/AA, 50/50    Comp 2   38.2                                                 Control (no additive)    48.5                                                 ______________________________________                                    

I claim:
 1. A substantially homogeneous copolymer consisting essentiallyof from about 5 to 90 mole percent acrylic or methacrylic acid units andfrom about 95 to 10 mole percent itaconic acid units and having a numberaverage molecular weight of from about 500 to
 7000. 2. The copolymer ofclaim 1 having from about 5 to 40 mole percent methacrylic acid unitsand from about 95 to 60 mole percent itaconic acid units.
 3. The alkalimetal, ammonium or amine salt of the copolymer of claim
 1. 4. An aqueoussolution containing the copolymer of claim 1 or 3 in the amount of fromabout 1 to 60 percent by weight.